The present invention relates to a room temperature curable resin composition for an aqueous paint, which contains substantially no volatile organic compound (hereinafter referred to as xe2x80x9cVOCxe2x80x9d) and which, while having a low minimum film-forming temperature (hereinafter referred to as xe2x80x9cMFT), forms a film having an excellent anti-blocking property and being excellent in various physical properties such as water resistance, smoothness and weather resistance.
In recent years, environmental regulations have become stricter with regard to reduction of toxicity and VOC, and also from the viewpoint of saving resources, it is demanded to use water in place of an organic solvent as a solvent for a paint in the field of paint industry. As a typical paint, an aqueous emulsion paint may be mentioned which employs an aqueous emulsion resin as a binder.
However, the aqueous emulsion resin has a specific MFT, and when the temperature of the surface to be coated was lower than MFT, it was necessary to blend an organic solvent as a coalescing agent for film-formation.
On the other hand, in order to prevent blocking which is likely to occur when coated plates are piled, it is necessary to use a resin of high hardness as a binder. However, a composition employing a resin of high hardness has a high MFT, and it is again necessary to blend a coalescing agent in a large amount. Thus, even an aqueous emulsion paint contained a substantial amount of VOC, and when drying of the coated film was inadequate, there was a problem that the anti-blocking property or water resistance tended to be poor due to the remaining VOC.
Further, with respect to an aqueous emulsion paint, a study has been made on a paint having both properties of a low temperature film-forming property and an anti-blocking property. As a result, as shown in JP-A-2-175742, JP-A-7-26196, JP-A-11-349828, etc., a paint having a good anti-blocking property has been developed which comprises an emulsion resin having carbonyl groups, having a relatively low MFT, and a compound having hydrazide groups, wherein particles are crosslinked to one another to improve the anti-blocking property. However, there has been a problem that, although a certain level of an anti-blocking property can be obtained only by such crosslinking of particles, the anti-blocking property is inadequate under severe conditions.
The present invention has been made in view of such problems of the prior art, and it is an object of the present invention to provide a resin composition for an aqueous paint which is excellent in the low temperature film-forming property and which forms a coating film excellent in the anti-blocking property and in the water resistance, smoothness, weather resistance, etc., without using VOC such as coalescing agent which is likely to cause environmental pollution or bad odor, or by an addition of a small amount of VOC, if used.
In order to accomplish the above object, the present inventors have conducted an extensive study and as a result, they have found it possible to certainly accomplish the above object by the following construction and have arrived at the present invention.
Namely, the present invention provides a resin composition for an aqueous paint, which comprises (a) a binder composed of different phase structure emulsion particles obtained by multi-stage emulsion polymerization, and (b) a compound having at least two hydrazide groups in its molecule, wherein the different phase structure emulsion particles have an outermost phase formed of an emulsion polymer which is an emulsion polymer of an ethylenic unsaturated monomer, having a glass transition temperature of from xe2x88x9250xc2x0 C. to 15xc2x0 C., and the following conditions (1) to (3) are satisfied:
(1) the emulsion polymer forming the outermost phase contains from 1 to 25 mass % of an ethylenic unsaturated monomer having a carbonyl group,
(2) at least one phase located inside the outermost phase of the different phase structure emulsion particles, is formed of an emulsion polymer of an ethylenic unsaturated monomer, having a glass transition temperature of from 50xc2x0 C. to 150xc2x0 C., and
(3) the binder has a minimum film-forming temperature of at most 30xc2x0 C.
Now, the present invention will be described in detail.
The binder (a) constituting the resin composition for an aqueous paint of the present invention is one composed of different phase structure emulsion particles prepared by multi-stage emulsion polymerization of an ethylenic unsaturated monomer in water.
The multi-stage emulsion polymerization is carried out by at least 2 stages, usually from 2 to 5 stages of emulsion polymerization of an ethylenic unsaturated monomer in water in accordance with a well known emulsion polymerization process, so that the emulsion polymer of an ethylenic unsaturated monomer thereby formed constitutes different phase structure emulsion particles comprising the outermost phase and one or more inner phases.
A typical example of the multi-stage emulsion polymerization process may be a process comprising a plurality of steps of repeating emulsion polymerization of an ethylenic unsaturated monomer at an elevated temperature of from 60 to 90xc2x0 C. in the presence of an emulsifying agent and a polymerization initiator, and further optionally a chain transfer agent and an emulsion stabilizer.
Examples of the emulsifier include anionic surfactants, such as an aliphatic acid salt such as sodium lauryl sulfate, a higher alcohol sulfuric acid ester salt, an alkylbenzene sulfonate such as sodium dodecylbenzene sulfonate, a polyoxyethylenealkylether sulfuric acid salt, ammonium polyoxyethylenephenylether sulfonate, polyoxyethylene polyoxypropylene glycol ether sulfuric acid salt, and a so-called reactive emulsifier having a sulfonic acid group or a sulfuric ester group and a polymerizable carbonxe2x80x94carbon unsaturated double bond in its molecule; nonionic surfactants, such as a polyoxyethylene alkylether, polyoxynonylphenylether, a sorbitan aliphatic acid ester, a polyoxyethylene aliphatic acid ester, a polyoxyethylene-polyoxypropylene block copolymer, and a reactive nonionic surfactant having the above-mentioned group and polymerizable carbonxe2x80x94carbon unsaturated double bond in its molecule; cationic surfactants such as, an alkylamine salt and a quaternary ammonium salt; and (modified)polyvinyl alcohol.
Examples of the polymerization initiator include those generally used for radical polymerization, and among them, a water-soluble material is preferable. For example, persulfates such as potassium persulfate and ammonium persulfate, azo type compounds such as 2,2xe2x80x2-azobis(2-amidinopropane)hydrochloride, 4,4xe2x80x2-azobis-cyanovaleric acid and 2,2xe2x80x2-azobis (2-methylbutaneamidoxime)dihydrochloride tetrahydrate, and peroxides such as aqueous hydrogen peroxide and t-butylhydroperoxide, may be mentioned. Further, a redox type compound including a combination of ferrous sulfate with a reducing agent such as L-ascorbic acid and sodium thiosulfate, is also useful.
Examples of the chain transfer agent include long chain alkylmercaptans such as N-dodecylmercaptan, aromatic mercaptans and halogenated hydrocarbons.
Examples of the emulsion stabilizer include polyvinyl alcohol, hydroxylethylcellulose and polyvinylpyrrolidone.
The emulsion polymerization is carried out by a one time-charging method of charging a monomer all at once, a monomer-dropping method of continuously dropping a monomer, a preemulsion method of previously mixing and emulsifying a monomer, water and an emulsifier and dropping the emulsified product, or a combination of these methods.
According to the present invention, in the preparation of different phase structure emulsion particles as a binder in accordance with the above-mentioned methods, as an ethylenic unsaturated monomer which is added at the final stage of multi-stage emulsion polymerization and forms the outermost phase, it is necessary to use an ethylenic unsaturated monomer having a carbonyl groups in an amount of from 1 to 25 mass %, preferably from 5 to 20 mass %, of the total ethylenic unsaturated monomers to form the outermost phase, in combination with such an ethylenic unsaturated monomer as to provide an emulsion copolymer of the outermost phase having a glass transition temperature (hereinafter referred to as xe2x80x9cTgxe2x80x9d) of from xe2x88x9250xc2x0 C. to 15xc2x0 C., preferably from xe2x88x9230xc2x0 C. to 10xc2x0 C. Also, it is necessary to use, for at least one of inner phases, such an ethylenic unsaturated monomer as to provide an emulsion copolymer having a Tg of from 50xc2x0 C. to 150xc2x0 C., preferably from 55xc2x0 C. to 100xc2x0 C.
Also, it is necessary to use a binder having a minimum film-forming temperature (MFT) of at most 30xc2x0 C., preferably at most 20xc2x0 C. The lower limit of MFT may, for example, be 0xc2x0 C., but the lower limit temperature is not limited to such a level.
By satisfying these conditions, it is possible to form a coating film without using or with use of only a very small amount of a VOC such as a coalescing agent, and it is possible to form a coating film excellent in the anti-blocking property or the water resistance.
In the present invention, Tg of an emulsion copolymer is one calculated in accordance with the following FOX formula.
1/Tg=W1/Tg1+W2/Tg2+ . . . +Wi/Tgi+ . . . +Wn/Tgn 
(In the above FOX formula, Tgi(K) represents the glass transition temperature of a homopolymer of each monomer constituting a polymer comprising n kinds of monomers, and Wi represents the mass % of each monomer, i.e. (W1+W2+ . . . +Wi+ . . . Wn=1.)
In the present invention, if Tg of the emulsion copolymer of the outermost phase of different phase structure emulsion particles is lower than xe2x88x9250xc2x0 C., a coating film obtained therefrom tends to be poor in pollution resistance and water resistance. On the contrary, if Tg exceeds 15xc2x0 C., film-formability tends to be poor at a low temperature.
Further, if MFT of the binder exceeds 30xc2x0 C., film-formability tends to be poor unless a coalescing agent is used in a large amount.
Now, the ethylenic unsaturated monomer used for forming different phase structure emulsion particles will be explained.
As described above, it is necessary for the ethylenic unsaturated monomer used for an emulsion copolymer forming the outermost phase to contain an ethylenic unsaturated monomer having a carbonyl group as the essential component. Such a monomer may, for example, be acrolein, diacetone (meth)acrylamide, formylstyrol, a (meth)acryloxyalkylpropanol, diacetone (meth)acrylate, acetonyl (meth)acrylate, acetoacetoxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate/acetylacetate, butanediol-1,4-acrylate/acetylacrylate, vinyl ethyl ketone or vinyl isobutyl ketone.
Among them, acrolein, diacetone acrylamide or vinyl ethyl ketone is particularly preferred.
If such an ethylenic unsaturated monomer having a carbonyl group is less than 1 mass % in the total ethylenic unsaturated monomer constituting the outermost layer, no adequate crosslinking among particles can be attained, and the anti-blocking property tends to deteriorate. On the other hand, if it exceeds 25 mass %, the water resistance, etc., tend to be poor, such being undesirable.
Further, various ethylenic unsaturated monomers commonly used in the preparation of acrylic resins can be used without any restriction as comonomers copolymerizable with the ethylenic unsaturated monomer having a carbonyl group.
Typical examples of such comonomers include (meth)acrylate type monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, pentyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, stearyl (meth)acrylate, xcex1-chloroethyl (meth)acrylate, cyclohexyl (meth)acrylate, phenyl (meth)acrylate, methoxyethyl (meth)acrylate, ethoxyethyl (meth)acrylate, methoxypropyl (meth)acrylate and ethoxypropyl (meth)acrylate; styrene type monomers such as styrene, methylstyrene, chlorostyrene and methoxystyrene; carboxyl group-containing monomers such as (meth)acrylic acid, crotonic acid, itaconic acid, itaconic acid half ester, maleic acid and maleic acid half ester; hydroxyl group-containing monomers such as 2-hydroxyethyl (meth)acrylate, 2(3)-hydroxypropyl (meth)acrylate, 4-hydroxybutyl acrylate and mono(meth)acrylic acid ester of allyl alcohol polyhydric alcohol; amide group-containing monomers such as (meth)acrylamide and maleinamide; amino group-containing monomers such as 2-aminoethyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, 3-aminopropyl (meth)acrylate, 2-butylaminoethyl (meth)acrylate and vinylpyridine; epoxy group-containing monomers and oligomers obtained by reacting an active hydrogen atom-containing ethylenic unsaturated monomer with an epoxy compound having at least 2 glycidyl groups, glycidyl (meth)acrylate or allylglycidyl ether; and N-methylol acrylamide having a N-methylol group, vinyl acetate, vinyl chloride, ethylene, butadiene, acrylonitrile, and a dialkyl fumarate.
As described above, these comonomers are used optionally in such a combination as to provide an emulsion copolymer forming the outermost phase having Tg of from xe2x88x9250xc2x0 C. to 15xc2x0 C. and a binder having MFT of at most 30xc2x0 C.
Further, as the ethylenic unsaturated monomer used for an emulsion copolymer forming at least one of inner phases of different phase structure emulsion particles, the same ethylenic unsaturated monomers as exemplified above can be used, but it is necessary to use them optionally in such a combination as to provide an emulsion copolymer having Tg of from 50 to 150xc2x0 C. and a binder having MFT of at most 30xc2x0 C.
It is particularly preferred that the emulsion copolymer of at least one of inner phases has an internal crosslinked structure, whereby MFT can be lowered without lowering the anti-blocking property.
The emulsion copolymer having an internal crosslinked structure can be produced by a method, such as a method of using a monomer having at least two polymerizable unsaturated double bonds in its molecule, such as divinylbenzene, ethylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate or allyl (meth)acrylate; a method of using a combination of monomers having functional groups which mutually react at a temperature during the emulsion polymerization reaction, for example, a monomer mixture selectively comprising ethylenic unsaturated monomers having a combination of functional groups, such as a carboxyl group and a glycidyl group, or a hydroxyl group and an isocyanate group; or a method of using a monomer mixture comprising silyl group-containing ethylenic unsaturated monomers, such as (meth)acryloxypropyl trimethoxysilane, (meth)acryloxypropyl triethoxysilane and (meth)acryloxypropylmethyl dimethoxysilane, which undergo a hydrolytic condensation reaction.
In the present invention, it is advisable to select and use a monomer so that MFT of the binder composed of different phase structure emulsion particles will be lower than MFT of a polymer obtainable by uniform emulsion polymerization of all monomers constituting the particles in one step.
Further, when a carboxyl group-containing monomer is used as an ethylenic unsaturated monomer, it may be neutralized with a neutralizing agent such as ammonia, dimethylethanolamine or triethanolamine.
The compound (b) having at least 2, preferably 2 or 3, hydrazide groups in its molecule, which constitutes the resin composition for an aqueous paint of the present invention, may, for example, be a carbohydrazide, oxalic acid dihydrazide, malonic acid dihydrazide, succinic acid dihydrazide, glutaric acid dihydrazide, adipic acid dihydrazide, sebacic acid dihydrazide, dodecanoic diacid dihydrazide, isophthalic acid dihydrazide, citric acid trihydrazide, 1,2,4-benzenetrihydrazide, or thiocarbodihydrazide. Among them, a carbohydrazide, adipic acid dihydrazide or succinic acid dihydrazide is preferred in view of the balance of e.g. the water resistance and the dispersibility to the emulsion.
The amount of the component (b) to be incorporated, is preferably an amount such that the hydrazide group will be from 0.1 to 2.0 equivalents, preferably from 0.3 to 1.2 equivalents, per equivalent of the carbonyl group in the emulsion copolymer forming the outermost phase in the component (a).
If the amount of the component (b) is less than the above range, the reaction with the carbonyl group in the emulsion particles (a) tends to be inadequate, whereby the anti-blocking property or the coating film hardness can hardly be attainable. On the other hand, if it is excessive, an unreacted component (b) will remain, and the water resistance, etc., tend to be poor.
The resin composition for an aqueous paint of the present invention comprises a binder composed of the above-described emulsion particles and a compound having a hydrazide group. This composition may be used simply in the form of a dispersion (emulsion) in water as a clear paint, but in order to impart various functions to a paint, it is preferable to incorporate various additives, such as a dispersant, an anti-settling agent, a thickening agent, an antiseptic agent, a UV ray-absorber, a photostabilizer, and a pigment such as a extender pigment, a coloring pigment or a rust-preventing pigment.
A paint obtained in this manner is applicable to various inorganic materials, metal materials, wood materials or plastic materials, and provides an excellent coating film by air-drying or force-drying at a temperature of at most 100xc2x0 C.